Non-shifting radiation source capsule

ABSTRACT

A substantially immobile &#39;&#39;&#39;&#39;non-shifting&#39;&#39;&#39;&#39; relatively low energy radiation source includes a particulate radioisotope bearing material positioned within a chamber formed in a capsule. Movement of the radioactive isotope element or any attenuating mass is prevented by a porous deformable conformational barrier member which has pores therein smaller than the smallest particle adjacent to the barrier and which prevents any attenuating producing movement of any particulate material within the capsule. In the case of gas producing radioisotopes, provision is made to permit escape of gas while preventing movement of attenuating masses or radioactive isotope material. Various radioisotopes and barrier members are described as well as various ways to assemble a &#39;&#39;&#39;&#39;non-shifting&#39;&#39;&#39;&#39; source.

o Umted States Patent 51 3,655,984 Dukes [451 Apr. 11, 1972 NON-SHIFTING RADIATION SOURCE Primary Examiner-Archie R. Borchelt CAPSULE Attorney-Marechal, Biebel, French & Bugg, William T. [72] Inventor: John R. Dukes, Worthington, Ohio Fryer m and Henry Peterson [73] Assignee: Industrial Nucleonics Corporation 1 ABSTRACT [22] Filed: Dec. 18, 1969 A substantially immobile non-shifting relatively low energy radiation source includes a particulate radioisotope bearing [21] Appl' 886338 material positioned within a chamber formed in a capsule. Movement of the radioactive isotope element or any attenuat- "25 25 84 ing mass is prevented by a porous deformable conformational 1h 5 barrier member which has pores therein smaller than the sm al- 58 Field oi Search ..250/84, 10 s "lest P r i sn w the barrier d which prevents y a tenuatmg producing movement of any particulate material I 56] mums Cmd within the capsule. In the case of gas producing radioisotopes, plOVlSlOll [5 made to perrmt escape of gas while preventing UNITED STATES PATENTS movement of attenuating masses or radioactive isotope v aterial. Various radioisotopes and barrier members are 2,830,190 4/1958 Karp ..250/l06 s m 3,515,875 6/1970 Keshishian ..250/l06 s $322 as ways assemble a cw sv wsv 25 22 20 I5 4| O I 3". :;'n I 0:13.05: a e "33" 0 i 2 v 2 I g5 3| 4o Patented April 11, 1972 3,655,984

I I INVENTOR JOHN R. DUKES A TT'ORNE'YS NON-SHIPPING RADIATION SOURCE CAPSULE BACKGROUND OF THE. INVENTION This invention relates to radiation source capsules, and more particularly to an improved source capsule which substantially eliminates changes in the emitted radiation due to shifting of the emitting material or because of movement of an attenuating material into a position within the capsule where it reduces the radiation emanating from the source capsule.

Reference is made to application Ser. No. 554,750, filed June 12, 1966, assigned to the same assignee, and now U.S. Pat. No. 3,488,502.

With an increased use of non-contacting measuring devices employing a radioactive source capsule, for example, radiation thickness gauges and the like, there has been an increased demand for increasing accuracy of the system which includes the source, the detector and the associated electronic and control equipment. By radioactive source is meant a radioactive isotope bearing material which emits radiation caused by the decay of the isotope. The radiation may be alpha, beta, gamma, neutron or other nuclear related radiation, or combinations thereof, and of varying energies. "Capsule as used in this description means supporting shell which may or may not be received in a support housing.

Radiation source capsules generally include a radioactive isotope bearing material supported within a shell, the shell including a relatively thin window through which the radiation passes. In some instances, materials are present in the source capsule which are frangible under conditions of vibration, pressure, or shock, or are adversely affected by the radiation emitted from the source, for example, organic materials, glass, silicates and carbon. While thepresence of these materials may not adversely affect the operation of the source capsule in some applications, there are circumstances under which small changes or shifts of the mass within the capsule may result in changes in attenuation within the capsule of the radiation emitted from the isotope resulting in what may be described as a shifting source," that is, a source whose transmitted radiation varies sufficiently to produce an erroneous indication of the variable being measured by the particular system, e.g., thickness, velocity, etc. The variation in transmitted radiation is different in its cause from the normally anticipated variations, for example, accumulation of dirt, etc., between the source and detector, and predictable gradual changes in emitted radiation due to natural decay of the radioisotope, each of which can be compensated for by any of several well known standardization procedures.

The conditions which give rise to a shifting source vary somewhat, for example, the source capsule in many cases is operated in an environment in which there is vibration and the like which may cause fracture of components within the capsule, particularly ceramic or other frangible materials. Also, the materials within the capsule may oxidize due to air and/or moisture which is sealed in the capsule, and the oxidation products may flake and may move into a position where they act'as attenuators of radiation, or if the products are radioactive the radiation they emit is attenuated differently.

in some instances, the radioactive isotope is of a type which results in the formation of a gas during radioactive decay, and accumulation of the gas within the capsule may cause movement or displacement of an attenuating mass in the radiation path, for example, the capsule window. In other cases, the radioisotope itself is particulate in form, and the particles thereof may change their relative position by what ordinarily would be considered a smallamount but which produces a noticeable change in radiation emanating from the capsule. Various radioisotopes have a high Z (atomic number) and thus exhibit relatively high self-attenuation. The physical displacement of a high Z radioisotope particle brings about a change in transmitted radiation, but if the displacement brings the particle in the radiation path of another particle, the first also acts as an attenuator for the radiation emitted from the other particle.

The presence of plastics in the capsule which are exposed to radiation results in crazing or cracking thereof over a period of time, while in other instances the presence of frangible materials such as carbon, glass or ceramic frits and silicates may result in the formation of small pieces as a result of vibration of the capsule or mechanical shock to the capsule. The size of such fragments, even though considered small under the usual circumstances, constitute movement of an attenuating mass which absorbs radiation within the capsule thereby changing the amount of radiation emitted from the capsule.

The changes in gamma radiation emanating from the capsule are particularly noticeable with those of relatively low energy, for example, the radioisotope Americium-24l. This particular isotope has a half life of approximately 462 years, and emits alpha radiation, in addition to gamma rays with an energy of 59.6 Kev, and other gammas of less significance. Amercium (Am) exists in various valence states and thus several different oxides are possible, although the dioxide (Am-241 O is frequently referred to, this material may be a mixture of two or more oxides. Thus, when the terms "Americium-24l oxide or Am- 241 O are used, these terms are intended to cover the dioxide, and the other oxides, and mixtures thereof. While Am metal exists, it oxidizes easily to the oxide which is a powdery material, and in fact the Am-24l O currently available is a particulate powdery material. Am-24l 0 is an alpha emitter. The alpha particle consists of two protons and two neutrons which is identical to the nucleus of a helium atom. By definition, one curie of activity produces 3.7 X 10 atoms disintegrating per second, and in the case of Am-24l 0 this results in the formation of approximately the same'number of helium atoms per second per curie of activity. The formed helium nucleons may easily pick up free electrons within the capsule to form helium gas. Am-24l 0 offers certain advantages as a gamma emitting radioisotope because of the energy level of the emitted gamma and the relatively long half life. The 59.6 Kev gamma emitted from Am-24l O is of an energy .level which renders it desirable for use in many commercial non-contacting and nondestructive measuring systems, for example, thickness gauges and the like. The fact that a gas is formed as a result of the radioactive decay, and the current availability of the isotope as a particulate Am-24l 0 powder creates some problems in providing an immobile non-shifting source. The fact that Am has a high atomic or Z number,i.e., 95, places it in the category of a high self-attenuator, a characteristic which tends to aggravate the problem of a shifting source.

U. S. Pat. No. 2,830,190 of Apr. 8, 1958 describes a structure in which a porous stainless steel plate is joined to a tube forming a cup-like structure which receives a radioactive salt. The porous stainless steel is used as a filter medium in the assembly of the radioactive salt into the cup, the salt being in the form of a slurry which must pass through the porous stainless steel. Then, the particles of the slurry are smaller than the pore openings in the stainless steel member.

U. S. Pat. No. 3,217,165 of Nov. 9, 1965, describes a standard source structure to be used as a calibration source, not normally exposed to the rough treatment generally encountered with sources used in gauging equipment. The source is in the form of a planchet in which an absorbent pad is adhered to the planchet and covered with a plastic disk. The stiffening disk is non-porous and the source material is absorbed in the absorbent.

The above referred to application describes a source structure which overcomes the difficulties associated with a shifting source. The present invention, while directed to the same problem,presents another solution to the same problem.

Accordingly, it is a primary object of the present invention to provide a radiation source capsule containing particulate material wherein changes in attenuation of emitted radiation due to source shifting is substantially eliminated.

Another object of the present invention is to provide a radiation source capsule wherein the radioisotope bearing material within the capsule includes a particulate solid radioisotope which results in the formation of gas during its decay, the radioisotope bearing material being maintained in a confined condition by a porous barrier member having pores smaller than the smallest particle of the radioisotope bearing material adjacent to the barrier to prevent changes in attenuation of radiation within the capsule by substantially eliminating shifts or movement of attenuating mass or radioisotope particles within the capsule while at the same time permitting passage of gas to a gas collection zone or reservoir within the capsule.

Another object of the present invention is the provision of a radiation source capsule which includes a substantially immobile non-shifting particulate radiation source material therein.

A further object of the present invention is to provide a radiation source capsule utilizing Americium-24l oxide in particulate form as the radioisotope, and wherein attenuating masses and radioisotope particles are maintained substantially immobile within the capsule by a porous barrier member having pore sizes smaller than the particles of the attenuating masses or radioisotope adjacent to the barrier while permitting passage of the gas formed by the decay of the radioisotope into a gas reservoir within the capsule.

SUMMARY OF THE INVENTION Radiation source capsules broadly include a supporting shell having therein a radioisotope bearing material. The shell includes a radiation transmitting window positioned to receive radiation from the radioisotope bearing material thereby permitting passage of radiation out of the shell. Positioned within the shell and on the side of the radioisotope bearing material opposite the window is a backer member. The objects above described of the present invention are achieved by positioning a porous barrier member between the backer and the radioisotope bearing material so that it substantially completely fills the space between the radioisotope bearing material and the backer member. The barrier is preferably composed of particulate ductile and formable materials such as aluminum, stainless steel, gold, platinum, silver, lead, and the like. The particulate materials are compressed using powdered metallurgy techniques to form a porous member in which the size of the pores is controlled and smaller than the smallest particle constituting the radioisotope material in the region adjacent to the barrier. The particulate material forming the barrier is maintained in conformational engagement with the radioisotope bearing material by the backer member. Positioned within the shell and spaced from the surface of the backer opposite the barrier is a sealing plug which is sealed to the shell. The spaced provided between the barrier and the plug forms a gas reservoir into which any fonned gases may pass.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an enlarged sectional view of a radiation source capsule in accordance with the present invention;

FIG. 2 is a view along the line 2-2 of FIG. 1 but in reduced dimension showing the back face of the backer; and

FIG. 3 is a sectional view of another embodiment of a radiation source capsule in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawing, the source capsule includes a supporting shell 12 preferably of a metal having high strength characteristics and being corrosion resistant, for example, stainless steel. In the form shown, the shell 12 is annular in shape, but other shapes may be used if desired. The shell 12 includes a radiation transmitting window 15 which is relatively thin to reduce attenuation of radiation emitted from a radioisotope bearing material generally indicated as positioned within the shell. The window 15 is preferably of stainless steel secured to the shell 12 as by welding or brazing and the like, although it may be integral with the shell, and of other material, if desired.

The radioisotope bearing material may include, for example, Cesium-l37, or Cobalt-60, both considered high energy gamma emitters; or beta emitters such as Strontium-90, Yttrium-90, Titanium-204, Ruthenium-106, Carbon-l4, or Promethium-l47; or alpha emitters such as Americium-24l, Polonium-2 l 0, or Radium-226. In the event a neutron source is desired, any alpha emitter may be used, in combination with a target material of high neutron yield. Other radioisotopes may be used, e.g., those providing direct neutron emission or other atomic and nuclear radiations, for example,positrons, etc. While a shifting source is not generally as much a problem with high energy materials as it is with low energy materials, there are certain instances in which the system accuracy requires a substantially immobile non-shifting relatively high energy radiation source within the capsule. A shifting source is, however, troublesome in the case of low energy gamma and beta sources, particularly in systems where the detector and the electronics have become sufficiently refined to be very sensitive to quite small changes in radiation emanating from the capsule for the reasons previously described.

For the purposes of illustrating the present invention, reference will be made to an Am-24l 0 source, which may be classified as a low energy gamma emitter, and which presents the problems of accommodating a gaseous product which results from the natural alpha decay and of exhibiting a high degree of self-attenuation. The radioisotope bearing material 20 preferably includes particulate radioactive isotope 22 and a bulking material 25, shown on a much enlarged scale for purposes of illustration and which are formed into a rigid matrix 30. It is desirable to use a bulking material since it makes it easier to get a more uniform distribution of the radioactive isotope in a wafer or layer of dimensions which are easier to handle. It is understood, however, that the matrix may be formed of powdered radioactive isotope without any bulking material, if desired, or it may be incorporated into ceramic beads or other binders. The bulking material may be a finely divided powder of metal or the like, for example, beryllium, carbon, aluminum, silicon, calcium, titanium, vanadium, chromium and the like proceeding progressively to materials of the high atomic number (2) such as platinum and gold. In the case of Am-24l, it is preferred to use a material having a relatively low Z number in order to minimize attenuation of the gamma rays by the bulking material in the matrix of radiation emitted from the radioactive isotope. Simultaneously it is desirable to avoid very low Z materials that provide a high neutron flux from the alpha bombardment. Typical low Z materials are carbon, aluminum, magnesium, silicon, copper and nickel, for example.

Received within the shell 12 and spaced from the surface 31 of the matrix 30 opposite the window 15 is a backer member 35, also shown in FIG. 2. The space between the backer member 35 and the surface 31 of the matrix is substantially completely filled by a barrier member 40 which includes a face 41 opposed to the face 31 of the matrix and which is in conformational engagement therewith. By substantially filling the space between the backer and the matrix, the barrier confines the components of the matrix, and by providing a conformational barrier, all surface irregularities in surface 31 are substantially filled thereby substantially eliminating any movement of the components of the matrix. Also the barrier prevents components of the backer from moving into the matrix region and thus prevents introduction of any undesired attenuators therein.

In one form, the barrier is in the form of particles of a ductile material such as aluminum, stainless steel, gold, silver, platinum, lead, and the like, compresses into place to provide a porous member whose pores are smaller than the smallest particle of the radioisotope bearing material. For example, and not to be construed as a limitation, if the material is capable of passing through a 60 mesh (U.S. Standard) screen but not an mesh screen, the particle size is less than 0.250 mm but greater than 0.177 mm. Thus, if the larger pore size in the barrier is 0.175 mm, then none of the particulate material in the radioisotope bearing material can pass through or migrate into the barrier. Since the barrier is composed of particulate material it is capable of conforming to any irregularities in surface 31 to prevent movement of particles of the radioisotope or any particulate bulking material which may be present. Moreover, being porous, the barrier permits passage into a space (to be described later) provided on the side of the backer 35 opposite the barrier.

It is also possible to preform a thin wafer outside of the shell, assemble it into the shell and press it into place thereby deforming the front face into conformational engagement with the opposed surface of the radioisotope bearing material. The advantage of this procedure is that the compacting pressure may be varied within wide limits thus providing closer control over the pore size of the porous backer. As a variation, the thin preformed wafer may be assembled and additional particulate material added over the exposed back surface, and compacted into place either before or after the addition of more material, or both. Whatever the procedure, the final result is to provide a barrier member whose front surface is in conformational engagement with the opposed surface of the material within the chamber containing the radioisotope hearing material.

Backer member 35 may be any material with sufficient mechanical strength to hold the barrier in place, e.g., carbon, ceramic, aluminum, and steel. Some of these materials are frangible under vibration and shock, and the resulting particles could migrate into the matrix unless their movement is prevented. The backer member is preferably metallic, for example, stainless steel.

In the form shown in FIGS. 1 and 2, the backer member 35 is a solid plug retained in position within the shell by a series of spot welds 44, for example, and the dimensions of the backer 35 are so coordinated and correlated with the inner dimensions of the shell as to provide a slight interference fit therebetween which provides a relatively tight mechanical fit while at the same time permitting passage of gas around the periphery of the backer in the areas between the spot welds 44. This can be accomplished for example by scoring the peripheral walls before engagement.

Positioned within the shell 12 and spaced from surface 47 of the backer is a sealing-plug 50 which is continuously sealed to the shell 12, for example, by a continuous weld or braze 52. The space 55 between the backer 35 and the sealing plug 50 forms a'gas reservoir into which any gas formed as a result of radioactive decay or chemical decomposition may expand. Since the capsule is virtually completely and hermetically sealed by the sealing plug 50, shell 12 and window 15, the progressive build-up of gas is vented from the area of the matrix to minimize gas accumulation, and resultant pressure build-up which may cause flexing or movement of the window 15. Since the window is a radiation attenuator, any movement thereof, even though slight, could result in a change in attenuation which is manifest as a change in radiation emanating from the capsule. Such movement would also create void space in the region between window 15 and barrier 40 which could permit particulate matter in the matrix region to move or shift and cause a variation in the emitted radiation intensity.

The radiation source capsule may be fabricated as follows: The window is assembled and fixed to the shell as by welding or brazing and thereafter a charge of particulate radioactive material is introduced into the shell. If desired, the charge may include a bulking agent, and the shell with the charge therein may be vibrated slightly to even the level of the charge. Thereafter, the barrier in the form of a particular powder may be placed into the shell. With the particulate material used to form the barrier in contact with the charge, and with the window supported on a suitable surface, a compacting pressure is applied to confine the components of the charge and to compress the barrier material to form a relativev ly rigid matrix from the loose particles and to compress the barrier material. The pressure may vary depending on the nature of the charge, the barrier material and the extent to which it is desired to compress the charge and barrier.

The compacting operation causes conformational engagement between the front face of the matrix and the opposed surface of the window as well as between surface 31 of the matrix and surface 41 of the barrier. If the backer 35 is used to apply the compressive force, there also tends to be some conformation between the facing surfaces of the barrier and backer which reduces the presence of small spaces and crevices into which particles may move. If not already in position, the backer is assembled and spot welded into place as indicated earlier. Thereafter, the sealing plug is assembled in position and sealed to the shell as previously indicated.

Referring to FIG. 3, another embodiment of the present invention is shown wherein like reference numerals have been applied where applicable. In this form, the particulate material in the chamber 52 formed between the window 15 and the backer 35 includes a layer 53 of particulate material between the radioactive isotope bearing material 20 and the porous barrier 40, the barrier being spaced from the radioisotope material. in this form, layer 53, which is a spacer may be particulate material such as ceramic or metallic particulate material which, along with the radioisotope bearing material, is immobilized by the porous barrier, thus, materials which are normally objectionable in a source because of possible movement are now not objectionable because they are effectively immobilized.

The material 20, of either FIGS. 1 or 3, may be a radioactive isotope encased in a refractory matrix, shown in U.S. Pat. No. 3,147,225, issued Sept. 1, 1964, for example.

The radioisotope bearing material 20 of FIG. 3 may be a mixture of particulate bulking material and radioactive isotope while layer 53 may be particulate bulking material or other particulate material. In this form, if the material 20 has a particle size less than 200 mesh (U.S. Standard) but greater than 325 mesh (U.S. Standard), then layer 53 should have pores no greater than about 0.040 mm and the barrier 40 should have pores smaller than the smallest particle making up the spacer layer 53. In this way confonnational engagement is assured at the interface between material 20 and 53 and between 53 and 40 to prevent any movement of radioactive isotope or any particulate material which can act as an attenuator.

It is also possible in accordance with the present invention to use a radioactive isotope in the form of a coated foil, for example Am-24l metal, or Am-241 coated on a suitable support. In either case, the Am-24l metal tends to oxidize within the capsule to form Am-241 O which in turn tends to powder. By providing a porous barrier and the gas reservoir as described, the isotope material is maintainedv confined and substantially immobile.

It is also possible in accordance with this invention to seal the backer to the shell as described previously and to use small weep holes through the backer which permit passage of gas to the reservoir. The presence of weep holes in the backer does not adversely affect the confining-action of the backer and barrier because the holes are sufficiently small to reduce the surface area of unsupported barrier surface to a minimum.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. In a radiation source capsule for use with gauges and the like, wherein said capsule includes a supporting shell and means forming a chamber within said shell for receiving a radioactive isotope, particulate radioisotope bearing material positioned within said chamber, said shell including radiation transmitting window means positioned to receive radiation from said particulate radioisotope bearing material and to permit passage thereof out of said shell, backer means positioned within said shell in adjacent spaced relation to said particulate radioisotope bearing material, the improvement comprising a conformable porous barrier means positioned within said shell between said backer means and said particulate radioisotope bearing material, said porous barrier means being compressed particulate material the pores in said barrier being smaller than the smallest particle adjacent to said barrier means, and said barrier means being in direct contact with the material in said chamber and substantially completely filling the space between said backer means and the said material for confining said particulate radioisotope bearing material and for preventing passage of particulate material between said backer means and said particulate radioisotope bearing material.

2. A radiation source capsule as set forth in claim 1 wherein said radioisotope bearing material includes a particulate radioisotope material and a particulate bulking material.

3. A radiation source capsule as set forth in claim 1 wherein said radioisotope bearing material is spaced from said barrier means by a particulate spacer layer, and wherein said porous barrier means includes pores smaller than both said radioisotope bearing material and said particulate spacer layer.

4. A radiation source capsule as set forth in claim 1 wherein said particulate radioisotope bearing material includes Americium-24l isotope, and wherein said source is a gamma source.

5. A radiation source capsule as set forth in claim 1 wherein said particulate radioisotope material includes a particulate refractory matrix the particles of which encase a radioactive isotope.

6. A radiation source capsule as set forth in claim 1 including gas reservoir means in said shell, said barrier means cooperating with said shell to permit passage of any gas produced by said radioisotope bearing material between said shell and said barrier means, and said backer means including means therein forming a gas passageway venting gas to said gas reservoir thereby preventing accumulation of gas pressure between said barrier means and said window.

7. A radiation source capsule as set forth in claim 1 wherein said radioisotope bearing material includes a radioisotope whose radioactive decay results in the formation of a gas, said barrier means being maintained in engagement with said shell by said backer means while permitting passage of the formed gas therethrough, means on the side of said barrier means opposite said radioisotope forming a gas reservoir for reception of the gas, and said backer means being so constructed and arranged as to permit passage of gas from said radioisotope to said gas reservoir.

8. A radiation source capsule as set forth in claim 7 wherein said backer means is a solid plug maintaining said barrier means in conformational engagement with the opposed surface of said radioisotope bearing material, sealing plug means positioned in said shell in spaced relation with said backer means, the space between said backer means and said sealing plug forming a gas reservoir, and said solid plug backer means cooperating with said shell to permit passage of gas from said radioisotope bearing material to said gas reservoir.

9. A radiation source capsule as set forth in claim 3 wherein said particulate radioisotope material includes a particulate refractory matrix the particles of which encase a radioactive isotope. 

1. In a radiation source capsule for use with gauges and the like, wherein said capsule includes a supporting shell and means forming a chamber within said shell for receiving a radioactive isotope, particulate radioisotope bearing material positioned within said chamber, said shell including radiation transmitting window means positioned to receive radiation from said particulate radioisotope bearing material and to permit passage thereof out of said shell, backer means positioned within said shell in adjacent spaced relation to said particulate radioisotope bearing material, the improvement comprising a conformable porous barrier means positioned within said shell between said backer means and said particulate radioisotope bearing material, said porous barrier means being compressed particulate material the pores in said barrier being smaller than the smallest particle adjacent to said barrier means, and said barrier means being in direct contact with the material in said chamber and substantially completely filling the space between said backer means and the said material for confining said particulate radioisotope bearing material and for preventing passage of particulate material between said backer means and said particulate radioisotope bearing material.
 2. A radiation source capsule as set forth in claim 1 wherein said radioisotope bearing material includes a particulate radioisotope material and a particulate bulking material.
 3. A radiation source capsule as set forth in claim 1 wherein said radioisotope bearing material is spaced from said barrier means by a particulate spacer layer, and wherein said porous barrier means includes pores smaller than both said radioisotope bearing material and said particulate spacer layer.
 4. A radiation source capsule as set forth in claim 1 wherein said particulate radioisotope bearing material includes Americium-241 isotope, and wherein said source is a gamma source.
 5. A radiation source capsule as set forth in claim 1 wherein said particulate radioisotope material includes a particulate refractory matrix the particles of which encase a radioactive isotope.
 6. A radiation source capsule as set forth in claim 1 including gas reservoir means in said shell, said barrier means cooperating with said shell to permit passage of any gas produced by said radioisotope bearing material between said shell and said barrier means, and said backer means including means therein forming a gas passageway venting gas to said gas reservoir thereby preventing accumulation of gas pressure between said barrier means and said window.
 7. A radiation source capsule as set forth in claim 1 wherein said radioisotope bearing material includes a radioisotope whose radioactive decay results in the formation of a gas, said barrier means being maintained in engagement with said shell by said backer means while permitting passage of the formed gas therethrough, means on the side of said barrier means opposite said radioisotope forming a gas reservoir for reception of the gas, and said backer means being so constructed and arranged as to permit passage of gas from said radioisotope to said gas reservoir.
 8. A radiation source capsule as set forth in claim 7 wherein said backer means is a solid plug maintaining said barrier means in conformational engagement with the opposed surface of said radioisotope bearing material, sealing plug means positioned in said shell in spaced relation with said backer means, the space between said backer means and said sealing plug forming a gas reservoir, and said solid plug backer means cooperating with said shell to permit passage of gas from said radioisotope bearing material to said gas reservoir.
 9. A radiation source capsule as set forth in claim 3 wherein said particulate radioisotope material includes a particulate refractory matrix the particles of which encase a radioactive isotope. 